Equalization of carrier systems



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EQUALIZATION OF CARRIER SYSTEMS Filed Jan. 28, 1928 4 Sheets-Sheet l Iii Le wet.

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EQUALIZATION OF CARRIER SYSTEMS 4 Sheets-Sheet 2 Filed Jan. 28, 1928 Evyaenzy in Elazycles M ata'qe Slgoes .fld asta 72 k E9 er 10- H I a a I H 2 I I u E'equemy in Elocycles flizyustable fatal/tigers 7 BY 5% J K ATTORNEY 4 Sheets-Sheet 4 A TTORNEY M. ILGENFRITZ Filed Jan. '28. 1928 Jan. 14, 1930.

EQUALIZATION OF CARRIER SYSTEMS 0 .6 W a... w W E m 21 AF w C co ll 0 w i m 00.2% J 3 Z e e C fi 7 im C DMZ h c 0 1:- m M nu H u 0 W 1 0 6 W m g Wm M m P o I i; q 2 FII: H1? C h w P. n W n 7. 1L a w in Z Mn w w m 4 A m h I T, B L w .1 a y m a MU why 2 A m ul 2 ML 2 mflm W 6 0( Patented Jan. l4, 593d LESTER BI. ILGEI Q'FRITZ, OF NEW ROCHELLE, NEW YORK, ASSIGNOB T0 RIGAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK EQUALIZATION OF CARRIER SYSTEMS Application filed January 28, 1928. Serial No. 250,194.

This invention-relates to the equalization of carrier systems so that all of the channels will be brought to substantially a common transmission level, and if desired, the common level may be maintained constant with changes in transmission conditions.

In a transmission system employing line conductors, the attenuation of the current varies with frequency, the attenuation increasing as the frequency goes up. This becomes a,very serious matter in a multiplex carrier system as the channels having the higher carrier frequencies will be much more attenuated than the channels transmitting at lower frequencies. If, in such a system, the

attenuation is plotted against frequency, the resultant curve will, within reasonable frequency limits, approximate a straight line having a slope depending upon the character of the circuit and the existing transmission conditions. This slope varies with different transmission conditions, and the variation in slope will, moreover, be different for different types of circuits.

Obviously, it is desirable that some form of compensation bemade so that the transmission at a given frequency be maintained at a desired level, and that some further compensation be made to bring all channels at i all times to that level. In other words, some compensation should be made so that the resultant slope will be horizontal, and will be maintained horizontal at a desired transmission level. I

In the case of a carrier telephone system, the term transmission level is generally understood to signify, not the absolute power in a-particular frequency band, but the relation between the power transmitted past any 7 point in the system and the power at some arbitrary reference point, usually at the transmitting end of the system.

Heretofore, it has been proposed to accomplish these results by transmitting a pilot frequency over the circuit,observing the level of the pilot frequency at one or more stations (usually at each repeater station and at the terminal), adjusting a potentiometer which treats all frequencies alike to compensate for the observed change in level at the pilot frequency, and at the same time making a level equalizing adjustment of some network in the circuit to compensate for the slope of the attenuation frequency characteristic assumed to exist at the particular pilot frequency amplitude observed.

Unfortunately, lines constructed with different gauges of wire and different types of insulators give different changes in slope for a given change in the over-all transmission equivalent. Furthermore, changes may occur in the gain of the repeaters due to variations in the battery supply, and as these changes in gain produce no change whatever in the attenuation frequency slope, it is evident that an adjustment of a'potentiometer over a. certain number of steps to bring the transmission level back to normal, accompanied by a compensating slope adjustment;

accompanied by difierent changes in loss at y the same frequency at different times. Such a condition obviously, makes the control of the equalizing slope according to changes in the over-all attenuation observed at a single frequency more or less unsatisfactory.

Accordingly, it is proposed, by means of the present invention, to so arrange matters that the e ualization or'slope correction be controlled y means of two pilot frequencies transmitted over the system, one or both of these frequencies serving also to control the adjustment of a potentiometer or other device for bringing the circuit back to a. normal transmission equivalent. By transmitting two pilot frequencies and observing the difference in the amplitudes at which they are received, the attenuation-frequency slope is determined. Suitable adjustments may be then made in an equalizer or slope corrector to bring the over-all frequency-attenuation 100 i derstood from the following detailed descrip-' curve for the system, including the equalizer, to a common horizontal level. This adjustment may be made either manually or automatically. An independent adjustment of the gain of an amplifier may then be made by means of a potentiometer or equivalent device to change the gain e ually at all frequencies to such extent as to ring the now common level of the circuit up to the normal level By this method of compensation the same equalizing and level adjusting apparatus may be used for all circuits in the plant, regardless of their character, and the adjustments will approximate an ideal condition under various conditions of any given circuit.

lhe invention will now be more fully untion when read in connection with the accompanying drawing in which Figure 1 shows a series of curves illustratingthe attenuationfrequency characteristics of'two difierent circuits under variable weather conditions; Fig. 2 shows a series of curves illustrating the changes in slope corresponding to the various conditions illustrated in Fi 1; Fig. 3 show s a series of curves illustrating how com'pen-- sating slope adjustments may be made by using equalizer networks having attenuationfr equency characteristics whose slopes pass through a common point at one frequency; Fig. 4 shows a similar set of curves by means of which the same slopes may be obtained but in which the curves do not pass through a] common level at one of the pilot frequencies; Fig. 5 illustrates a type of network which may be used for obtaining any of the desired slopes illustrated in Figs. 3 and 4; Figs. 6 and 6, taken together, illustrate a circuit ar-- range'ment for carrying out'the principles of the invention; Figs. 8 and 9 illustrate two different forms of adjustable equalizers which may be used in connection with the invention;

while Fig. 7 illustrates a modified form of pilot receiver apparatus enablingv simultaneous adjustment of the potentiometer and slope compensator.

Referring to Fig. 1, let us suppose we have a transmission line which we will call-M,.and that we transmit over this line several frequencies each having the same level at the transmitting end. If we measure the received current at the several frequencies under dry weather conditions, it will be found that the attentuation loss with frequency will progressively increase, as shown by the curve M of Fig. 1. Under wet weather conditions,

the attenuation will be greater at all frequencies, but the increase in attenuation will be greater for the higher frequencies, as indicated by the curve M of Fig, l. The curves M and M have difierent slopes, and at intermediate conditions between wet and dry, the

same circuit may have the attenuation curves indicated at M and M, of Fig. 1, each of these curveshaving still different slopes.

retain-i Now, if we observe the transmission loss for some particular frequency at a given instant, it is possible to adjust'a potentiometer and increase the ain to bring the transmission level at that lrequency up to the level at which it started, or to any other level that maybe desired. If, furthermore, thecon struction of the line remains unaltered and no changes occur in the repeaters due to battery fluctuations, or the like, it will be possi ble to associate with each step ,of adjustment of the otentiometer a correspondin step in the adJustment of a network or com 'ination of networks by means of which-the proper compensating changes will be made for the .other frequencies involved, so that all freuencies will be adjusted to the same level. n other words, a network may be out into the circuit each timea compensating change is made in the gain of a repeater, which network will; have an attenuation frequency characteristic whose slope will he just the reverse of that of the line, so that the combined slope of the network and the line will be horizontal. This involves the assumption that cated at N N N, and N of Fig. .1. If 'we assume that the pilot frequency, whose loss is measured in order to determine the potentiometer and otheradjustments, be 20 kilocycles, for example, it will be evident that while the attentuation measurement will enable us to adjust a potentiometer to'control the level at that particular frequency, the corresponding slope compensating networks which were used'in the case of line M will not fit the present'case at all. In fact,an inspection of the curves at Fig. 1 shows that a knowledge of the transmission loss at 20,000 cycles, for exam le, gives us no information whatever as to t 1e slope compensation which should be made, as the slope will vary from that of the curve M to that of the curve N even under the same transmission conditions.

Furthermore, let us suppose we are measuring the loss at 20 kilooycles over the line N under dry weather conditions, and that due to some abnormal condition of a repeater, (which changes the gain at all frequencies alike), we get a measured attenuation loss of 20 units instead of 12 units. The observer will then, knowing he is working with the line N, assume that he should make a slope compensating adjustment corresponding to the curve N, of Fig. 1.. Now, the facts are that under these conditions the slope would be the dry weather slope represented by curve N the only difference due to the repeater wearer being that the loss at all the frequencies involved would be increased eight units, so

that the actual attenuation frequency curve,

for this particular condition would be as represented by the curve M of Fig. 1. curve M while drawn for the line M, is parallel to the curve N and happens to accurately represent the assumed abnormal condition on the line N Obviously, if we make a slope adjustment corresponding to N it will be incorrect as the slope adjustment should correspond to M (or N It is therefore clear that measurements'of the attenuation loss at one frequency will not afford sufficient data from which todetermine both the necessary potentiometer adjustment and the necessary slope compensating adjustment. Therefore, in accordance with the present invention, it is proposed to make measurements of the attenuation loss of the circuit at two frequencies. This will give data from which we will know at any time the slope of the attenuation frequency curve and the actual attenuation loss at each frequency over the range with which we are concerned,-this regardless of the nature or character of the circuit over which the measurements are made, and regardless of changes either in the character of the'individual elements of the system or in the external conditions which affects the transmission of the circuit.

For example, ifin Fig. 1 we use 15 kilocycles and 25 kilocycles as the pilot frequencies and measure the attenuation loss at each of those frequencies, such attenuation measurements fix at once the slope of the attenuation frequency curve for the circuit at that time. If we make measurements at these two frequencies and find that at 15 kilocycles the loss is 9 units, and at 25 kilocycles the loss is 18 units, we know the-- s ope for which compensation must be made is that represented by the line N If, on the other hand, the observed attenuation losses are 16 units and 25 units, respectively, we

know that the slope for which compensation must be made is that of the curve M which, as it happens, is the same as that of the curve N It is unnecessary to learn anything about the nature of the line over which'the measurements are being made. or anything about the weather conditions. The two measthe over-all attenuation frequency character-- istic of the circuit. Consequently, the slope The or equalization adjustments may be made quite independently of the over-all transmission level adjustment The various slopes represented b the curves M M etc., and N N etc., of ig. .1

are shown in their relationship to each other in Fig. 2, where each of the curves is drawn with the same slope as in Fig. l, but through same slope as the curve M of the line M. 1

Therefore, so far as bringing all of the chan-' nels to the same level is concerned, it will be necessary to have adjustable networks capable of producing compensating slopes corresponding to those slopes numbered 3, 6, 9, 10, 11 and12 of Fig. 2 in order to take care ofthe several conditions illustrated in Fig.1 for the two lines M and N.

Assuming that the compensating arrangements are to correct for intermediate weather conditions not indicated in Fig. 1, or that they, are to be used with still other lines having different characteristics, it might be desirable to have compensating adjustments varying by frequent steps all the way from slope 1 to slope 14 of Fig. 2.

A convenient way to obtain the desired range of slope adjustment would be to have a set of networks, each so designedas to have an attenuation-frequency characteristic corresp onding'to a different one of the curves of Fig. 3. These networks may be of the ,ty e illustrated in Fig. 5 and would preferably be a common level at one of the pilot frequencies. The characteristics of this type of network are discussed at length in the patent to Zobel, No. 1,603,305 of October 19, 1926. If desired, instead of using a separately designed not-- work for each slope, a series of identical networks, each having a slope indicated by the curve a: of Fig. 4, may be designedand, by connecting different numbers of these networks in tandem, various slopes illustrated in Fig. 4 may be obtained.

In the foregoing discussion, it has been assumed that the curves are straight lines, as indicated in Fig.

1. While this is approximately true over the limited range of frequencies employed for all of the channels of the carrier system transmitting in one direction, in actual practice there is a slight curvature, in these lines over the entire useful frequency range. This does not introduce any material complication, however, as the curves all havethe same general form, and the networks making up the adjustable equalizer can be designed and, in

attenuation-frequency.

' so designed that their slopes all pass through practice, are designed to take care of the slight amount of departure from the straight line involved at each slope for which compensation is to be made.

Figs. 6 and 6 when placed side by side, illustrate schematically a circuit arrangement for carrying out the invention. The two terminal stations A and B are shown with an intermediate repeater at station C, the repeater being connected with the terminal station by line sections L and L respectively. Itwill be understood, of course, that in practice any desired number of intermediate repeater stations will be provided.

At station A, the various carrier transmitting channels are combined through band filters such as BR, (only one of which is shown) in a transmitting amplifier TA the various frequencies used for transmitting then passing through a directional filter WE to the line.- The receiving channels are branched from the line through a similar directional filter EW... The pilot apparatus at 4 station A comprises two generators G andgGg for producing the pilot frequencies f and f2 which are passed through pilot filters or selecting devices PF and PF respectively.

ing apparatus of known type will be employed. Also there will be provided 'at the terminal station B and atv suitable intermediate points, such as repeater stations, pilot apparatus for controlling the adjustments of the potentiometer and frequency equalizer. For example, at station B, the carrier channels pass through directional filters WE to a common receiving amplifier RA from the output of which the carrier channels are separated into their respective terminal circuits through band filters as BF only one'of which .is illustrated. The gain of the amplifier RA is controlled by means of an adjustable potentiometer'E'P as will be-descrihed later. An adjustable equalizer EZ for compensating for the slope of the attenuation-frequency characteristic of the received range of frequencies is also interposed in the circuit. The nature and operation of these devices will be described in moredetail later. The transmitting apparatus at station B is not illustrated. it being understood that this apparatus will be similarto that at station A and that the various channels (including pilot frequen ,cies for transmission in the reverse direction) are combined in the commontransmit-ting amplifier Til and pass to the line through the directional filter EW At the repeater station C, the two line sections L and L, are connected by two paths,

one for transmitting in each direction. The east-west path selects all the channel fre quencies transmitted in that direction by may be similar to the corresponding equalizer at station E. Similarly, the east-west path includes a common amplifier RA controlled by the potentiometer WP The path also includes an equalizer "WZ similar to the equalizer EZ r In* order to adjust the potentiometer El at station C,-one of the pilot frequencies f is selected by means of the filter PF and is transmitted to an amplifienrectifier AR to produce a direct current proportional to the amplitude of the received pilot frequency. This current operates a relay R5 hen the transmission conditions are normal at th s pilot frequency, the armature of the relay R is adjusted so that it will be in a neutral position. If, however, the transmission level departs from normal, due to change in weather conditions or what not, the arma-' ture will be shifted to one or the other of its back contacts, depending upon whether there is an increase or decrease in received current. This operates the mechanism PZM to shift gg adjustable switch of the potentiometer As the potentiometer and control mechanism at station C are in detail similar to the potentiometer ER, and control mechanism PZM at station B, it will be obvious that the operation of the armature of the relay R will energize one of the relays corresponding to X and'Y of Fig. 6,, thereby operating the motor corresponding to PZM in either a forward or reverse direction, depending upon which way the armature of the relay, is act-'- ing. This motor is mechanically arranged to set the switch of the potentiometer EP to any desired position. The switch is therefore-shifted until the gain of the amplifier is increased or decreased to such a point that the received pilot frequency is restored to its normal amplitude.

Since the potentiometer EP consists of resistance steps, it produces the same change in gain at all the carrier frequencies involved. In order to compensate for the difierence in the level of the various channels, or, in other words, in order to compensate for the slope of the attenuation-frequency characteristic, the adjustable equalizer EZ is arranged to be controlled by a control mechanism ZCM similar to that already described. Frequencies ii and 2 are selected by the pilot filters PF I quencies as received at station C. The pull upon the armature will therefore be a measure of the slope of the attenuation-frequency curve for the circuit extending from station A to station C and will be independent of the absolute amplitudes of the two pilot frequencies. Y The control mechanism ZCM, will, of course, be similar to that illustrated in Fig. 6 at PCM and will automatically adjust the switch of the equalizer EZ. to bring all of the channels to the same level as will be described later. It will also be understood that arrangements will be provided in connection with the east-west path of the repeater to control the adjustment of the potentiometer WP and the equalizer WZ through similar pilot frequencies transmitted in theopposite direction. These arrangements will, of course, be in all respects, similar to those described in the west-eastpath. So, also, at station B, the received pilot frequency 7, will actuate the rela R to operate the control mechanism PC b to adjust the potentiometer EP Likewise, the pilot frequencies f, and willbe combined in opposition at station to operate the relay B in a manner similarly described in R at station C. The re,- la R will actuate the control mechanism Z M to adjust the equalizer EZ In the arrangement above described, the pilot apparatus at station C produces the necessary eqaulization and potentiometer adjustments to compensate for the variable transmission losses between station A and station 0, while the corresponding apparatus at stationnB performs a similar function with respect to the section of the circuit extending from station C to station B.

Where the adjustable equalizers such as V p v tiometer, the ad ustment of the potentrome- EZL and EZ are to function to accordance with the principles'discussed in connection with Fig. 3, such equalizer may be made up as illustrated in Fig. 8. Here a switch S may be adjusted to any one of a number of contact positions, the number of contact positions being for shown as comprising fourteen. Each of these contact positions corresponds to one of the curves 1, to 14:, inclusive, of Fig. 3. Switching relays 1 to 14, inclusive, are associated with the contacts 1 to 14 of the controlling switch for the purpose of sw tching the purpose of illustration,

individual networks N to N into the line clrcult. Each of these networks will be differently designed so that each will have the attenuation-frequency characteristic whose slope will correspond to a different one of the curves of Fig. 3. i

The effect of the adjustment of switch S ofFig. 8 is to successively bring different networks into the circuit until a network is .selected whose slope is just complemental to that of the existing slope of the line'cir- -cuit. When the proper network has been selected, both pilot frequencies will be received over the line and throu h the net: work at the same level so that t e relay R or R as the case may be, will have its armature restored to its neutral position and further adjustment of the equalizer will not take place.

Where the slopes of all of the networks N to N inclusive, pass through a common point at one of the pilot frequencies or, .in

other words, the loss introduced by each of the networks is the same at one pilot frequency different networks may be switched into or out of the circuit without changing the amplitude of that pilot frequency asreceived. As shown in Fig. 3, the networks are all designed so that they have a similar loss at, for example, 35 kilocycles. If this figure is to be used as the frequency f of the system, as above described, the adjustment of the equalizer will have no eflect upon the amplitude of the pilot frequency f There- 'fore, if the actual amplitude of the frequency f as received is above or below normal, the relay R at station C and the relay R at station B, as the case may be, will operate its corresponding control mechanism to adjust the potentiometers EP and EP respectively. This'results in raising or lowering the then existing level at thepilot frequency f, to a desired normal level.

Briefly stated, the operation of the equalizer at each station is to bring all of the channels to a common level as received at that station. The potentiometer at that station is also adjusted so as to raise or lower the common' level to a preassigned normal level for the system. Since the adjustment of the equalizer does not affect the amplitude of the ilot frequency which controls the potenpensating slope from different combinations of networks of identical design. For example, as shown in Fig. 4, each network might be designed to have a slope indicated 14, inclusive. The relay contacts are so arranged that' if none are energized, all of the networks N to N inclusive are connected in tandem over the back contacts of the relays. The network N is permanently connected to the line terminals at the left. Whenever a relay is operated, all of the networks to the left thereof are disconnected from the networks to the right and are connected directly over the front contacts of the relay to V the line terminal at the right.

' terminal at the right. If the switch is moved For example, if it is desired to compensate for the slope indicated at 1 in Fig. 2, the switch S will be moved toposition 1 thereby energizing relay 1 which disconnects network N from all the-remaining networks and connects said network N directly to' the line to position 2, relay l is deenergized and relay 2 is actuated. This results in disconnecting network N from the networks to the right thereof and connecting it to the lineterminal at the right. Network N is, of course, connected in tandem with network N by release of relay 1. This'combination. will compen- 'sate for the slope shown at 2 in Fig. 2. If

the switch is moved to position 13, relay 13 is actuated, all of the relays to the left of 13 being deenergized. All of the networks N to N inclusive are connected in tandem.

The relay 13 disconnects the networks from network N and connects them directly to the line terminals at the right. This gives a combination which will compensate for the S10 e 13 of Fig. 2.

s above described, the operation of ad usting the potentiometer and equalizer. are accomplished automatically. In some cases, however, it may be desired to ad ust the switches of the potentiometers and equalizers manually. For example, at the terminal station in Fig. 6 the switches 3, 4 and 5 may be opened. The control mechanism will now cease to function and the attendant will merely read the meter M and adjust the switch of the equalizer EZ' until the read ing of the meter is normal. The meter M i will then be read and the switch of the potentiometer adjusted until the meter M indicates a normal reading. The adjustment of the equalizer brings all channels to the same level so that pilot frequencies have the same amplitude. The adjustment of the posponding to 3, 4. and 5 in Fig. 6 may be opened and the adjustment of the potentiometer andequalizer at station 0 be accomplished manually from the readings of the meters M and M In general, of course,

where the adjustments are to be made manually, the station nearest the source of pilot frequencies would make the adjustments first,

so that from each station the pilot frequen 'cies, as'they are sent out, towards the next.

station, will have the same amplitude. If this were not done and one of the stations further along the line as, for example, the terminal station should attempt to make the 3dr justment first, such terminal station would necessarily have'to make adjustments for the entire change in level throughout the system. Notwithstanding this adjustment at the terminal, however, the slope would still be abnormal'at intermediate stations such as O and if the attendant at such intermediate station should thereafter make a compensation to render the slope horizontal, the slope would be correspondingly changed at the ter-, nnual and would have to be readjusted there.

It will be clear that when the system operates in accordance with the principles of Fig. 4, the'equalizer adjustment will change the loss at the pilot frequency f, and hence change the received amplitude of the pilot frequency j, which controls the potentiometer.' In order to avoid any interference between the operations of adjusting the potentiometer and the equalizer, the pilot receiving'circuit may be arrangedas in Fig. 7. Here the pilot frequencies f, and f are separately impressed upon the amplifier-rectifier units AR and AR which compri'seyacuum tubes whose output circuits are connected in a push-pull relation. The. two windings of the relay R02 are connected, respectively in the output circuits of the devices AR and AR Therefore, the armature of the relay R will respond to the difference between the amplitudes of the frequencies f and f as received. The relay R functions to actuate the control mechanism to adjust the equalizer until the slope of the overall attenuation-frequency characteristic becomes horizontal.

The potentiometer controlling relay R in-efiect, the losses at this frequency will determine the setting of the potentiometer. Such potentiometer setting may take place simultaneously with the setting of the equal izer' without the one interfering with the other. I

It will be understood, of course, that where the system is to be operated according to the principles illustrated by the curves of either Fig. 3 or Fig. 4, the apparatus of Fig. 7 may be employed at both the repeater station and the terminal station.

It will be obvious that the general principles herein disclosed may be employed in many other organizations widely different from those illustrated without departing from the spirit of the invention as defined in the following claims.

What is :laimed is:

1. In a multi-frequency transmission system comprising a plurality of line sections connected in tandem, the method of equalization of the transmission over a range of frequencies, which consists in transmitting two pilot frequencies over the entire system, observing the amplitude of each of the pilot frequencies after transmission over each sec.- tion to determine the slope of the attenuationfrequency characteristic of each section, and introducing losses into each section which vary with frequency in accordance with curves whose slopes are, complemental .to those of the observed attenuation-frequency characteristics of the several sections, to

make the transmission equivalent of any section of the system the same for all frequencies. l I

2. In a multi-frequency transmission system comprising a plurality of line sections connected in tandem, themethod of obtaining a constant transmission equivalent for a range of frequencies, which consists intransmitting two pilot frequencies over theentire system, observing the amplitude of each of the pilot frequencies after transmission over each section to determine the slope of the attenuation frequency characteristic of each section, introducing losses into each section which vary with frequency in accordance with curves whose slopes are complemental to those of the observed attennation-frequen cy characteristics of the several sections, to make thetransmission equivalent of any section of the system the same for all frequencies, and adjusting the gain of an amplifier in each section in accordance with the observed ampliti de of at least one of said frequencies after passing over each section.

3. In a multi-frequency system, a plurality of line sections connected in tandem,

means to transmit twopilot frequencies over all of the sections of said system, means associated with each section to indicate the amplitude of each of said pilot frequencies after transmission over each section to determine to transmit two pilot frequencies over all of I thesections of said system, means associated with each section to indicate the amplitude of each of said pilot frequencies after transmission over each section to determine the slope of the attenuation-frequency characteristic of each section, means to interpolate 1n each section a combination of networks, each combination having a slope to compensate for the indicated slope of the corresponding line section, an amplifier in each section,

and means to adjust the gain of each amplifier in accordance with the observed amplitude of at least one of said pilot frequencies after passing over the section corresponding to the amplifier.

5. In a multi-frequency transmission system I whose attenuation increases with frequency, means to transmit two pilot frequencies over said system, means to indicate theamplitude of each of said pilot frequencies after transmission to determine the slope of the attenuation-frequency characteristic of the system, a plurality of networks the attenuation of each of which decreases with increase in the frequency, and means to interpolate said networks in said system in such combination as will compensate for the observed slope of the attenuation-frequency characteristiciof the system.

6. In a multi-frequency transmission system whose attenuation increases with frequency, means to transmit two pilot frequencies over said system, means to indicate the amplitude of each of said pilot frequencies after transmission to determine the slope of the attenuation-frequency characteristic of the system, a plurality of networks the attenuation of each of which decreases with increase in the frequency, means to interpolate said networks in said system in such combination as will compensate for the observed slope of the attenuation-frequency characteristic of the system, amplifying meansassociated with said system, and means to adjust the amplifying means in accordance with the observed amplitude of at least one of said pilot frequencies.

7 7. In a multi-frequency transmission system whose attenuation increases with frequency, means to transmit two pilot frequencies over said system, means to indicate the amplitude of each of said pilot frequencies after transmission to deterine the slope of the attenuation-frequency characteristic of -the system, an equalizer including a plurality of networks, each of said networks havingan attenuation which varies with frequency but which is the same for all of said networks at one of said pilot frequencies, and means to interpolate said networks in said system in such combination as will compensate for the observed slope of the attenuation-frequency characteristic of the system.

8. In a multi-frequency transmission system whose attenuation increases with frequency, means to transmit two. pilot frequencies over said system, means to indicate the amplitude of each of said pilot frequencies after transmission to determine the slope of the attenuation-frequency characteristic of the system, an equalizer including a plurality of networks, each of said networks having an attenuation which varies with frequency but whichis the same for all of said networks at one of said pilot frequencies, means to interpolate said networks in said system in such combination as will compensate for the observed slope of the attenuatiomfrequency characteristic of the system, amplifying means associated with said system, and means to adjust said amplifying means in accordance with the observed amplitude of at least one of said pilot frequencies.

9. In a multi-frequency transmission sys tem Whose attenuation varies with frequency,

an adjustable equalizer adapted to be adjusted to compensate for the variation of the attenuation of the system with frequency, an

adjustable transmission device for raisingand lowering the transmission level of the system, means to transmit two pilot frequencies over said system, means to indicate the difference between the amplitudes of the pilot frequencies after "transmission, means to control said adjustable equalizer in accordance with the indicated difference, means to indicate the sum of the amplitudes of the two pilot frequencies after transmission, and means to control said adjustable transmission device in accordance with the indicated sum. In testimony whereof, I have signed my name to this specification this 27th day of January, 1928. I r

LESTER M. ILGENFRITZ. 

